A typical vehicle occupant restraint system for a vehicle comprises a belt system (often comprising a lap belt portion and a shoulder belt portion) anchored to structural parts of the vehicle such as the floor, a pillar such as a B pillar or to a seat frame. Where the restraint system includes a shoulder belt portion (also referred to herein as a “shoulder harness”), the shoulder harness typically extends through a web guide that is coupled to a structural part of the vehicle such as the B pillar or the seat frame. The restraint system further includes a retractor, often called an “ELR” or emergency locking retractor, which is designed to take up or provide a controlled amount of slack in the belt system when the belt system has been fitted about a vehicle occupant. The retractor allows payout of belt (typically the shoulder belt) in response to movement of the vehicle occupant during normal vehicle operation. However, at the onset of a crash, the retractor becomes automatically locked, to restrain further belt payout, and to keep the vehicle occupant in her or his seat.
The prior art also teaches vehicle occupant restraint systems with tensioning devices, also referred to as “belt tighteners”, and/or load-limiting devices, to better control the manner in which force(s) are applied to a vehicle occupant during a crash. Specifically, a tensioning device applies a positive force to the belt immediately at the onset of a crash, to tighten the restraint against the vehicle occupant. A load-limiting device acts between the belt or retractor and a structural part of the vehicle and dissipates force applied to a vehicle occupant during a crash. When a tensioning device/belt tightener and load-limiting device are combined into a vehicle occupant restraint system, the tensioning device reduces residual slack in the seat belt system and the load-limiting device controls the subsequent dissipation of force on a vehicle occupant during the crash.
U.S. Pat. Nos. 6,183,015 B1, 6,196,589 B1 and 6,209,916 B1 teach known types of load limiting devices. In those patents a load limiting device includes a hydraulic piston with fluid chambers on its opposite sides, and a flow control system for controlling flow to one or both chambers, to control movement of the piston under forces applied to the piston by a vehicle occupant restraint system. Moreover, in those patents a crush tube is provided, and is crushed by the piston as the piston moves in one direction in the fluid chamber, to further dissipate forces applied to the piston by a vehicle occupant restraint system.